A protection relay is a microcontroller based intelligent electronic device with a basic function to protect electrical equipment by tripping a circuit breaker and interrupting a power line in case of over current or earth fault situations. The tripping signal on behalf of a trip coil or other actuator of the circuit breaker can be generated by the protection relay when, for example, the measured current in the line exceeds a nominal or preset value for a predefined time period. In certain situations such as Ring Main Unit (RMU) installations in urban areas, a self-supplied relay may for example be used. The self-supplied protection relay utilizes energy from the current sensing transformers to supply power to the relay electronics circuit and to supply energy to operate trip coils. The design of a self-supplied relay can have several constraints associated with it to ensure that the measurements are accurate and sensitive, and that its circuitry is efficient and optimized for power consumption. Some of these constraints and methods to generate power supply by controlled charging are disclosed in the WIPO publication WO 2009101463.
A self-supplied relay may be customized by a user through mechanical binary or Dual In-line Package (DIP) switches for parameter setting, or alternatively through battery-power alphanumeric LCD based Human Machine Interfaces (HMI). The HMI may be a detachable component as disclosed in the WIPO publication WO2009071454. The detachable HMI offers greater flexibility for the user in adaptation, configuration and display.
The DIP switches based HMI has limited functionality, restricted for example to binary combinations achievable for given DIP positions, apart from the constraints that a user has to do for decoding the switch position through a table to understand the interpretation of switch positions with respect to functionality being configured.
An HMI having features such as using push buttons or a touch screen interface can enhance the primary function of the product through an interactive user interface by displaying the various parameters of the relay including the line current measurements, protection settings, Event logs, and so forth. Such a HMI can also facilitate fine tuning of the achievable settings through given DIP combinations.
As one may recognize, a presence of a HMI can increase the scope of interaction that is possible with the relay and therefore the design of the HMI should be capable for scaling and support of functionality as desired for effective operation and configuration. This makes the design of such a HMI for a protection relay non trivial and specifically, for the self-powered relay, it involves optimized design for power supply and utilization (e.g., includes processing of information for display) to enable functions of the HMI even at trip conditions for the purposes of relay configuration and display of information.